<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="nav">
    <div>aaa</div>
    <div>bbb</div>
    <div>ccc</div>
</div>
<script>
    var o = document.getElementById("nav");
    console.log(o);
    console.log(o.children[2].previousElementSibling);
</script>
</body>
</html>